Heat Index Calculator
Find out how hot it really feels by combining air temperature and relative humidity into the heat index.
Example
At an air temperature of 90°F with 60% relative humidity, the Rothfusz regression gives a heat index of about 100°F — it feels roughly 10°F hotter than the thermometer reads, putting you in the "Extreme caution" zone.
How it works
Uses the NWS Rothfusz regression: a 9-term polynomial in temperature (°F) and relative humidity (%), with low-RH and high-RH adjustments. For mild conditions (heat index under ~80°F) it falls back to the simpler Steadman average to avoid overestimating.
Good to know
The Heat Index Calculator turns two numbers you can read off any thermometer and hygrometer — air temperature in degrees Fahrenheit and relative humidity as a percentage — into the apparent "feels like" temperature your body experiences. It is useful for anyone planning time outdoors in warm weather: gardeners, runners and cyclists, coaches and parents, outdoor and construction workers, and event organizers who need a quick read on how oppressive the air actually is.
Reach for it when the dry-bulb reading alone does not tell the whole story. Humid air slows the evaporation of sweat, so 90°F at 60% humidity strains the body far more than 90°F in a dry climate. The calculator returns the heat index in both Fahrenheit and Celsius, shows how many degrees it sits above the actual air temperature, and tags the result with a color-coded risk band: Safe, Caution, Extreme caution, Danger, or Extreme danger.
Read the result alongside that badge rather than the raw number in isolation. The band thresholds mirror the National Weather Service categories — roughly 91°F and up is "Extreme caution," 103°F and up is "Danger" — and the "Above actual" figure tells you at a glance how much the humidity is amplifying the heat. A reading at or below the air temperature simply means dry conditions are letting your sweat work efficiently.
One practical caveat: the underlying formula assumes shade and a light breeze, so direct sun can add a significant amount on top of what you see here, and the index becomes meaningful mainly at 80°F or above. Use it as a screening estimate for planning hydration and rest breaks, not as a substitute for an official forecast or medical judgment in vulnerable people.
Frequently asked questions
Why does the calculator sometimes show a value lower than the air temperature?
At low humidity and moderate temperatures, sweat evaporates efficiently, so it can actually feel cooler than the dry-air reading. The NWS formula reflects this with a low-humidity correction, and the simpler Steadman average is used below about 80°F where the full regression is unreliable.
How accurate is the heat index formula?
The Rothfusz regression reproduces the official NWS heat index table to within about ±1.3°F. It assumes shade and a light wind; in direct sunlight the real 'feels like' temperature can be up to 15°F higher.
Is my data uploaded anywhere?
No — this calculator runs entirely in your browser. Your inputs never leave your device, and it works offline once loaded.
Is this calculator free?
Yes, completely free with no sign-up and no limits.
People also ask
What is a dangerous heat index level?
The National Weather Service generally flags a heat index of about 103°F and above as the 'Danger' range and 125°F and above as 'Extreme danger.' Values from roughly 91°F to 103°F fall in 'Extreme caution.' Risk rises with prolonged exposure and physical activity.
What is the difference between heat index and temperature?
Temperature is the air reading from a thermometer, while the heat index combines that temperature with relative humidity to estimate how hot it actually feels to the human body. When humidity is high, the heat index is higher than the air temperature; in dry air it can even be lower.
Is heat index the same as feels like temperature?
In warm weather they are effectively the same thing — 'feels like' is the everyday label many forecasts use for the heat index. In cold weather, 'feels like' refers instead to wind chill, which is a different calculation based on temperature and wind speed.
How do you calculate heat index from temperature and humidity?
The standard method is the NWS Rothfusz regression, a polynomial equation that takes air temperature in Fahrenheit and relative humidity in percent and applies low- and high-humidity adjustments. For milder conditions a simpler averaging formula (the Steadman average) is used to avoid overestimating.
Does heat index account for direct sunlight?
No. The heat index assumes shaded conditions with a light wind. Standing in full sun can raise the effective feels-like temperature by a substantial amount above the calculated value.
What humidity makes heat dangerous?
There is no single cutoff, but higher relative humidity raises the heat index sharply at warmer temperatures because sweat evaporates less effectively. For example, at 90°F the difference between 40% and 70% humidity can push the heat index from the caution range into the danger range.
Can the heat index be lower than the actual temperature?
Yes. At moderate temperatures with low humidity, sweat evaporates efficiently and the air can feel cooler than the thermometer reads, so the calculated heat index comes out below the actual air temperature.
At what temperature does heat index matter?
The heat index is most meaningful at air temperatures of about 80°F (27°C) or higher. Below that threshold the full regression formula is unreliable, and a simpler estimate is used instead.
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